1. Field of the Invention
The present invention relates to nonreciprocal circuit devices, and more particularly, to a nonreciprocal circuit device, such as an isolator or a circulator, used in microwave bands.
2. Description of the Related Art
Nonreciprocal circuit devices, such as isolators or circulators, transmit signals in a predetermined direction and forbid transmission of the signals in an opposite direction. Using this characteristic, isolators are used in transmission circuit sections for mobile communication devices, such as automobile telephones and cellular phones.
An example of such a nonreciprocal circuit device includes a nonreciprocal circuit device disclosed in Japanese Unexamined Patent Application Publication No. 2006-135419. The nonreciprocal circuit device is a two-port isolator including a ferrite, permanent magnets, a circuit substrate, and a yoke. Furthermore, first and second central electrodes are arranged on the ferrite such that the first and second central electrodes are isolated from each other and intersect with each other. For example, as shown in FIG. 10 (the nonreciprocal circuit device shown in FIG. 10 is slightly different from the nonreciprocal circuit device disclosed in Japanese Unexamined Patent Application Publication No. 2006-135419, is merely illustrated as a comparative example used to facilitate a comparison with the nonreciprocal circuit device of the present invention, and is not a known nonreciprocal circuit device), electrodes 35c to 35e and electrodes 36i to 36p are provided on an upper surface 32c and a lower surface 32d of a ferrite 32. Conductive films 35a and 35b of a first central electrode 35 are arranged on first and second main surfaces 32a and 32b, and conductive films 36a to 36h of a second central electrode 36 are arranged through insulating films 37 and 38 on the conductive films 35a and 35b. The conductive films 35a and 35b are connected to each other through the electrode 35c so as to define the first central electrode 35. One end of the first central electrode 35 is connected to the electrode 35d (terminal A), and the other end of the first central electrode 35 is connected to the electrode 35e (terminal B). Moreover, the conductive films 36a to 36h are connected to one another through the electrodes 36i to 36k and electrodes 36m to 36p so as to define the second central electrode 36. One end of the second central electrode 36 is connected to the electrode 35e (terminal B) and the other end of the second central electrode 36 is connected to an electrode 36l (GND).
In the isolator described above, to obtain a small insertion loss by performing matching of the input impedance, the first central electrodes 35 and the second central electrodes 36 must intersect each other with predetermined intersection angles θ1 and θ2 as shown in FIGS. 11A and 11B. Various conditions must be considered in order to minimize the insertion loss, and the intersection angles θ1 and θ2 should be less than predetermined angles.
However, in the first central electrode 35 and the second central electrode 36, since the conductive films 35a and 35b are arranged on an inner side relative to the conductive films 36a to 36h of the second central electrode 36, when the intersection angles θ1 and θ2 are small, gaps G1 to G4 generated between the conductive films 35a and 35b and the electrodes 36p, 35e, and 36i become small as shown in FIGS. 12A and 12B, and accordingly, defect occurs due to short circuiting. Therefore, when the gaps G1 to G4 having sufficient sizes are provided, the size of the ferrite 32 in a vertical direction (short side) is increased, and accordingly, the size and height of the isolator cannot be sufficiently reduced. That is, with this configuration, the reduced intersection angles θ1 and θ2 (matching of input impedance and low insertion loss) are not obtained while the sufficient gaps G1 to G4 are maintained to prevent defects due to short circuiting. Consequently, the size and height of the device cannot be sufficiently reduced. Furthermore, the device cannot be efficiently used with a high frequency of about 1 GHz or more, because, as an operation frequency increases, the intersection angles θ1 and θ2 must be reduced.